1. Field of the Invention
The present invention relates to an oil-free scroll compressor and, more particularly, it relates to an oil-free scroll compressor including a plurality of scroll compressing mechanism blocks, with an object of increasing an overall of the oil-free scroll compressor.
2. Description of the Prior Art
The oil-free scroll compressor has been used to supply an oil-free compressed gas, typically, an oil-free compressed air. In one type of the oil-free scroll compressor, the scroll compressing mechanism block for compressing a gas comprises a fixed scroll and an orbiting scroll, each having an end plate and a spiral wrap upstanding from the end plate, with the scrolls being assembled together so that their axes are in staggered relation from each other and their wraps are in meshing relation with each other. A motor is provided to produce an orbiting motion of said orbiting scroll relatively to the fixed scroll, without producing a rotation of the orbiting scroll about its own axis, whereby a gas is compressed in a compressing space formed between the orbiting scroll and the fixed scroll and the compressed gas is discharged to outside through a central discharge port formed in the end plate of the fixed scroll. Such construction of the scroll compressing mechanism block itself is substantially the same as that of a conventional oil-lubricated type scroll compressor. An example of the oil-free scroll compressing mechanism block of this type is disclosed in Japanese Patent Application Laid-Open No. Sho 63-179185.
In another type of the oil-free scroll compressor, the scroll compressing mechanism block for compressing a gas comprises two rotating scrolls, each having an end plate and a spiral wrap upstanding from the end plate, with the scrolls being assembled together so that their axes are in staggered relation from each other and their wraps are in meshing relation with each other. A motor or motors are provided to rotate the scrolls in a same direction and at a same speed, whereby a gas is compressed in a compressing space formed between the scrolls and discharged to outside through a central discharge port formed in the end plate of one of the scrolls. The construction of the scroll compressing mechanism block is substantially same as that of the conventional oil-lubricated scroll compressor of this type. An example of the oil-free scroll compressing mechanism block of this type is disclosed in Japanese Patent Application Laid-Open No. Hei 2-245486.
In the oil-free scroll compressor, no lubricant oil is supplied to the compressing space formed between the scrolls and, therefore, small clearances are held between the wraps of these scrolls and between the top of the wrap of one of the scrolls and the bottom surface opposed thereto of the other scroll. Otherwise, coating layers having self-lubricating property, such as coating layers of ethylene tetrafluoride resin, molybdenum disulfide or the like, are formed on the portions of the scroll which may come into contact with other parts during operation. Such coating layers are also formed on the sliding portions of the end plates of both scrolls.
The present invention is not directed to the construction of the oil-free scroll compressing block itself; however, to facilitate an understanding of the present invention, an explanation will be given to the construction of the oil-free scroll compressing block before describing the present invention.
Referring to FIG. 1, the oil-free scroll compressing mechanism block of the type disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 2-245486, comprises a first scroll 1 having an end plate 1a and a spiral wrap 1b upstanding from said end plate, said end plate la including a pulley portion 1c integrally formed therewith. The oil-free scroll compressing mechanism block further comprises a second scroll 2 having an end plate 2a and a spiral wrap 2b upstanding from said end plate, said end plate 2a including a pulley portion 2c integrally formed therewith. A pulley 3 and a pulley 4 are fixedly mounted on a driving shaft 5, and the pulley 3 and the pulley portion 1c of the first scroll 1 are connected together by means of a timing belt 6 engaged therearound, while the pulley 4 and the pulley portion 2c of said second scroll 2 are connected together by means of a timing belt 7 engaged therearound. A chamber for housing the above elements is formed by frames 8, 9 and 10 which are connected together. The frames 8 and 9 are formed with bosses 8a and 9a respectively which project inwardly of said chamber. The first scroll 1 and the second scroll 2 are rotatably supported by said bosses 8a and 9a through radial bearings 11 and 12, respectively, and the wraps of the scrolls 1 and 2 are held in meshing relation with each other.
The axis of the boss 8a that is the axis of the first scroll 1, is arranged in staggered relation to the axis of the boss 9a that is the axis of the second scroll 2, with a distance therebetween which corresponds to a predetermined radius of rotation, so that a compressing space is formed between the wraps of both scrolls.
A thrust bearing 13 is arranged between the frame 8 and the end surface of the first scroll 1. A thrust bearing 14 and an elastic member 15 are arranged between the frame 9 and the end surface of the second scroll 2.
An air suction pipe 16 is connected to an inside cavity 8b of the boss 8a. Communicating passages 1d are formed in the end plate 1a of the first scroll 1 to communicate the inside cavity 8b of the boss 8a with a compressing space formed between the peripheral portions of the scrolls. A discharge port 2d formed at the center of the end plate 2a of the second scroll 2 communicates with an inside cavity 9b of the boss 9a and the inside cavity 9b is connected with an air discharge pipe 17. The above mentioned driving shaft 5 is supported by the frame 10 through bearings.
The shaft 5 is driven by a motor (not shown) and the rotation of the shaft 5 is transmitted through the pulleys 3 and 4, the timing belts 6 and 7 and the pulley portions 1c and 2c to the first scroll 1 and the second scroll 2, thereby producing the synchronized rotation of these scrolls in the same direction and at the same speed. Thus, air is conducted from the suction pipe 16 through the inside cavity 8b of the boss 8a and the communicating passages 1d to the compressing space formed between the scrolls 1 and 2. The air compressed in the compressing space is passed through the central discharge port 2d of the second scroll 2 and the inside cavity 9b of the boss 9a and discharged from the discharge pipe 17.
The conventional oil-free scroll compressor comprises a single scroll compressing mechanism block as described above and a single motor for driving said scroll compressing mechanism block. Generally speaking, in the construction of a scroll compressor, as the diameter of the scroll is increased, for example to a size larger than 300 mm .phi., a gas pressure applied to the end plate of the scroll is increased to an excessively high value and a vibration of the scroll caused by an imbalance of weight of the scroll is also increased to an excessively high value, so that it is technically difficult to manufacture a scroll compressor having a large diameter. Under these circumstances, the oil-free scroll air compressor which is currently manufactured is limited to a small compressor having a capacity lower than 3.7 KW.
On the other hand, a screw type compressor has been used as an oil-free rotary compressor. However, the screw type compressor is rather suitable for use as a compressor having a capacity higher than intermediate capacity, and it is actually difficult to produce an oil-free screw compressor having a capacity lower than 15 KW because it has a lowered performance and an increased temperature of discharged air.